In some manufacturing operations, usually in the final stages of production, bending operations are performed upon metallic or plastic structural members. The bending process is a complex process which seeks to avoid stress concentration of the points of bending. This is usually accomplished through a combination of stretching and bending across a preformed die.
The way that the workpiece is bent and stretched depends upon the shape of the workpiece and the sequence of bends to which it is subjected. Some of the modern workpiece stretching presses are computer controlled in order to give repeatable results in the bending process.
However, even these machines must be set up on a trial and error basis, and several workpieces must be destructively experimented upon in order to get first approximations of the ideal bending sequence.
Furthermore, even before this trial and error approach can be carried out, the bending die over which the workpiece is stretched must be present. The design of the bending die very nearly approximates the void which is surrounded by the finally shaped workpiece. Since the workpieces normally have a moment of elasticity, they tend to try to assume a shape having a radius of bend of lesser extent than was desired. This condition is known as springback and is defined as the elastic recovery of metal after a stress has been applied. The shape that the workpiece springs back to is almost never the shape optimally desired, nor the shape which conforms to the bending die. Reworking the workpiece to mitigate the effects of springback have been known to increase the unit cost of production by 50%.
Given the above limitations, one method has been to cause the workpiece to become excessively bent in the bending direction, such that upon springback the workpiece will assume the proper shape. This method invites the die designer to guess what the shape of the bending die should be. Even if a bending die designer were fortunate enough to guess the proper shape of the bending die for one particular type of workpiece, a slight change in workpiece construction can cause an improper springback. Such a change in workpiece might result from a change in suppliers.
In addition, all previous efforts at attempting to solve the springback problem was directed toward circumferential part configurations. This was due to the mathematical complexity of the problem. Although 70% to 80% of the frames formed are circumferential, significant utility advances in the ability to handle more complex shapes would be a most welcome addition to the part forming art.